Elevated concentrations of oxidizable constituents, in particular, of NO.sub.x, CO, H.sub.2, and hydrocarbons, can occur in the exhaust gases of spark-ignition and diesel engines, internal-combustion machines and incineration plants, e.g. as the result of a component malfunction such as an injection valve or as the result of incomplete combustion. To optimize the combustion reaction, it is therefore necessary to determine the concentration of these exhaust gas constituents. A method for measuring oxidizable gases is described in Unexamined Japanese Patent Application No. 60-61654, in which a stoichiometric reaction with oxygen takes place at a first measuring electrode made of platinum-class metals, and quasi-equilibrium states are established at one or more additional metallic measuring electrodes with reduced catalytic activity for the oxygen equilibrium reaction. Nernst voltages E1 and E2 are measured between the measuring electrodes and a reference electrode, which is exposed to a reference gas having a constant oxygen partial pressure, and the concentrations of the gas constituents calculated from the difference between these voltages on the basis of calibration curves.